1. Field of the Invention
The present invention relates to the field of fiber optics, and more specifically, to the splicing of low-temperature multi-component glass fibers with silica glass fibers.
2. Description of the Related Art
In the field of fiber optics, joining or splicing of optical fibers is a well-known and widely practiced technique. The most common method for splicing of two standard fused silica fibers is based on the fusion of the adjacent ends of the optical fibers that are to be joined. The fibers are brought close to each other and are aligned so that their cores are coaxial with each other. Heat is transferred to both fiber ends by an electric arc or filament between two electrodes that are positioned on either sides of the axis of the two optical fibers. This heat is sufficient to soften the glass at the end of each of the two fibers to be joined. The optical fibers are then brought in contact and the hardening of the softened glass as the temperature is lowered below the softening and glass transition temperatures to form a permanent bond between the fibers. See, for instance, D. L. Bisbee, xe2x80x9cSplicing Silica Fibers With an Electric Arcxe2x80x9d, Applied Optics, Vol. 15, No. 3, Mar. 1976, pp. 796-798. These techniques have been designed for and used to fuse fibers that have the same or very similar material compositions, e.g. two standard silica telecom fibers, in many applications including erbium doped fiber amplifiers (EDFAs).
In many applications, two fibers having different glass compositions and substantially different softening temperatures must be fusion spliced. Typically, a special fiber of some sort is being fusion spliced to a standard silica telecom fiber. The standard fusion splicing process must be modified to accommodate the difference in softening temperatures and provide a low loss ( less than 0.2 dB) and mechanically reliable fusion splice.
A. Barnes et al., xe2x80x9cSapphire fibers: optical attenuation and splicing techniques,xe2x80x9d Vol. 34, No. 30 Applied Optics, Oct. 20, 1995 pp. 6855-6858 discloses a suitable method for splicing sapphire fiber to silica fiber. Silica fiber is a glass, isotropic, amorphous solid with a softening temperature of about 800-1000xc2x0 C. as composed in Barnes"" experiments. Sapphire, on the other hand, is a single anisotropic crystal with a melting temperature of over 2000xc2x0 C. Initial tests fusing Sapphire directly to silica produced a strong splice but showed evidence of mullite crystal formation, which resulted in unacceptably high optical losses, 10 dB. To prevent mullite formation, the sapphire fiber was coated with silica by chemical vapor deposition (CVD) and fusion spliced as before. Although this prevented mullite formation, the optical losses were still high due to alignment issues. To improve fiber alignment, a capillary-tube splice technique was used.
Y. Kuroiwa et al., xe2x80x9cFusion Spliceable and High Efficiency Bi2O3-based EDF for Short-length and Broadband Application Pumped at 1480 nm,xe2x80x9d Optical Fiber Communication, Optical Society of America, February, 2001, discloses a method of fusion splicing a bismuth oxide (Bi2O3) based Er doped fiber (Bi-EDF) to a silica telecom fiber. The ends of the Bi-EDF and SiO2 fibers are heated by arc discharge using a conventional fusion-splicing machine. The arc discharge time is precisely controlled to control the fusion condition of Bi-EDF, which has a softening temperature lower than the SiO2 fiber. The two fiber ends are pushed toward one another to achieve fusion splicing.
The Asahi Glass Company (AGC) conducted and published an extensive study xe2x80x9cTechnical Bulleting: Bismuth-based EDFxe2x80x94A Broadband, High Efficiency and Compact EDFxe2x80x9d on the effectiveness of different glasses to provide compact EDFAs and concluded that Bismuth based glass provided the best overall properties. A key factor in this determination was AGC""s ability to form mechanically reliable low-loss fusion splices between Bismuth Oxide fibers and silica fibers and their inability to form such splices with Tellurite, Fluoride and Phosphate glasses, which have lower glass transition and softening temperatures then Bismuth based glass. AGC used an arc discharge at the gap between the fibers to form the fusion splice. However, Bismuth based glass does not provide the gain per unit length or other spectroscopic properties of Tellurite or Phosphate.
There remains an industry need for a method of fusion splicing low-temperature multi-component glasses such as Phosphate and Tellurite to standard silica fibers for use in compact EDFA and other telecom applications.
In view of the above problems, the present invention provides a low-cost approach for providing a low loss and mechanically robust fusion splice between a standard silica fiber and a low-temperature multi-component glass fiber such as phosphate, germanate or tellurite.
This is accomplished with an asymmetric configuration for fusion splicing the fibers. Instead of placing the heating element at the gap between the two fibers, the heating element is moved along the silica fiber a distance do from the gap. This asymmetric configuration heats but does not soften the silica fiber and heats the multi-component glass fiber to above its softening temperature directly via the heating elements and indirectly via the silica fiber. Thus, the temperature at the end of the silica fiber Tsi is greater than the temperature at the end of the multi-component fiber Tmc. This temperature gradient serves to improve thermal diffusion between the two fibers when brought into contact thereby strengthening the fusion splice. Either electrode arc or resistive heaters can be used to fuse the fibers. In either case, the heat is preferably localized onto the silica fiber, which reduces the direct heating of the multi-component fiber, to maximize the temperature gradient and increase bond strength.
The addition of an outer cladding layer can be used to further strengthen the fusion splice of a phosphate or germanate glass fiber or to enable fusion splicing of other even lower temperature glass compositions such as tellurite. The multi-component glass fiber is drawn with an outer cladding that is chemically and thermally compatible with both the multi-component glass fiber and silica fiber (xcx9c100% SiO2). More specifically, the material for the outer cladding will be a different multi-component glass having a softening temperature that is higher than that of the first multi-component glass but close enough so that the two materials can be drawn together in a fiber without any crystallization. The cladding material will also exhibit a glass network that is similar to that of silica fiber in order to form strong thermal diffusion bonding. For example, phosphate or germanate fibers may be drawn with a silicate outer cladding and a tellurite fiber may be drawn with a phosphate outer cladding. As a result, optical performance is dictated by the core and inner cladding layer of the first multi-component glass fiber while mechanical performance of the fusion splice is determined by the outer cladding. The ability to draw, rather than deposit, the outer cladding greatly simplifies the process, which lowers cost.